Process For The Production Of Anhydrides

ABSTRACT

In a process for producing anhydrides in a tubular reactor, gaseous reactants are passed downwardly through at least one reactor tube to a catalyst carrier where they pass into a passage defined by an inner perforated wall of a catalyst container before passing radially through the catalyst bed towards a perforated outer container wall. The reaction occurs as gas contacts the catalyst. Unreacted reactant and product exits the container though the perforated outer wall and then upwardly between an inner surface of a container skirt and the outer wall of the container and then over the end of the skirt and caused to flow downwardly between the outer surface of the skirt and the inner surface of the reactor tube where heat transfer takes place. These steps are repeated at any subsequent catalyst carrier product is then removed from the reactor outlet.

The present invention relates to a process for the oxidation ofhydrocarbons in the presence of a catalyst to form anhydrides. Moreparticularly it relates to a process for the catalytic oxidation ofbutane or benzene to maleic anhydride or the oxidation of o-xylene,naphthalene or mixtures thereof to phthalic anhydride.

Processes for the oxidation of hydrocarbons such as butane and benzeneto maleic anhydride are well known in the art. For about 50 years, themain process route to maleic anhydride has been the oxidation of benzenein the vapour phase and this remains a commercial route accounting foraround 15 to 20% of global capacity. However, high benzene costs meanthat the process is becoming less attractive from an economicstandpoint.

As the chemistry of the reactions are very similar, many of the oldplants intended to utilise benzene as a starting material have beenretrofitted to enable n-butane to be used as the starting material.Whichever starting material is used, the reaction is very exothermic.However, as discussed in more detail below, the energy released from thereaction in which n-butane is the starting material exceeds that wherebenzene is the starting material and this is reflected in the steamco-product.

It should be understood that there are a number of side reactions. Inparticular, there are mechanisms that result in small amounts of aceticacid and acrylic acid being formed in addition to carbon oxides.

The chemistry for the oxidation of butane and benzene to maleicanhydride are very similar as are the catalysts used in the reaction.Typical commercial catalysts are based on unsupported vanadiumphosphorus oxide (VPO). These are conventionally used in fixed bedreactors. Promoters such as those selected from lithium, zinc andmolybdenum may also be used. Recent research has suggested thatmagnesium, calcium and barium ions may also be effective promoters withthe potential to generate higher conversion, yield, and selectivity thanthe unmodified VPO catalyst.

The ratio of phosphorous to vanadium in the catalyst determines theactivity of the catalyst and hence the life. Catalyst activity isgreater at high phosphorous:vanadium ratios, but the catalyst life issacrificed as the activity increases. A phosphorous:vanadium ratio of1.2 has been suggested to provide the optimal balance between activityand catalyst life.

The process for the production of maleic anhydride is illustratedschematically in FIG. 1. In this process, feed is prepared in section A.Here the benzene or n-butane feed is vaporised and mixed with compressedair in a controlled manner generally using static in-line mixers. If acatalyst promoter, such as trimethyl phosphate, is to be used, it willbe added at this stage. The prepared feed is then passed to section Bwhere the oxidation reaction takes place.

The oxidation will generate a substantial amount of heat which must beremoved. Careful control of the reactor is required to prevent thehydrocarbon/oxygen composition breaching combustion limits.

The product of the oxidation reaction is then removed from section B andpassed to section C where recovery of the crude product occurs. Heremore heat is removed from the reaction off-gas and then an aqueous orsolvent based recovery system is generally employed to obtain the crudemaleic anhydride stream. This crude stream is then passed to section Dwhere product purification occurs. This is typically a two-stepdistillation approach involving light ends removal and the separation ofa heat cut. This separation is generally vacuum assisted.

Energy recovery is a key factor in achieving an economic process for theproduction of maleic anhydride. Some or all of the reactor off-gases,post maleic anhydride recovery, non-condensables from vacuum systems,tank vent gases, and waste hydrocarbon rich liquids may all be utilisedin energy recovery in section E. Generally these are incinerated torecover energy as super-heated steam. The steam can be used to supplyenergy to the process or to supply energy to integrated processes.

Specifically, the oxidation of n-butane to maleic anhydride is a complexreaction in which the catalytic system abstracts 8 hydrogen atoms andinserts 3 oxygen atoms giving an exothermic heat release of 297kcal/g-mol of butane, with the main side reaction of butane oxidation tocarbon oxides releasing a further 500 kcal/g-mol of heat. Thesereactions are shown by the equations below:

CH₃CH₂CH₂CH₃+3.5O₂→C₂H₂(CO)₂O+4H₂O

CH₃CH₂CH₂CH₃+5.5O₂→CO₂+2CO+5H₂O

For the oxidation of benzene to maleic anhydride the heat releases are352 kcal/g-mol and 555 kcal/g-mol:

C₆H₆+4O₂→C₂H₂(CO)₂O+2H₂O+CO₂+CO

C₆H₆+6O₂→3CO₂+3CO+3H₂O

However, it should be understood that the ratio of carbon monoxide tocarbon dioxide is not fixed

On-going development work to improve the maleic anhydride productionprocess centres on identifying more active catalysts and obtaining anincrease in yield. However, the benefit of more active catalysts can bedifficult to realise in, for example, fixed bed reactors unless thespecific reaction rate can be increased. Such increases in productivitygenerate larger amounts of heat which must be removed at a rate thatmaintains a stable operating temperature.

Similar processes are used for the oxidation of o-xylene, naphthalene ormixtures thereof to phthalic anhydride. These processes are also wellknown in the art. The catalyst used will generally be similar to thoseused in the production of maleic anhydride or may be modificationsthereof based on a vanadium-titania mixture. For the oxidation ofo-xylene there is a significant heat release of 265 kcal/g-mol, as wellas there being heat released when naphthalene is oxidised (467kcal/g-mol):

C₆H₄(CH₃)₂+3O₂→C₆H₄(CO)₂O+3H₂O

C₁₀H₈+4.5O₂→C₆H₄(CO)₂O+2H₂O+2CO₂

Generally the reactions are carried out in specialised reactors whichinclude devices to provide cooling. Generally fixed bed or fluid bedreactors will be used. However, it is difficult to realise the benefitsof more active catalysts unless the specific reaction rate can also beincreased.

Currently the best heat removal to catalyst ratio for a fixed bed systemis delivered by an axial tubular reactor. In this arrangement, catalystpellets are loaded inside tubes of an axial reactor. Cooling medium,such as vaporising water, is supplied around the tubes. Reactant gasesare then passed through the tubes where they contact the catalyst andthe oxidation reaction takes place. The heat evolved is transferredthrough the tube wall to the surrounding cooling medium. In view of theneed to control the heat within the tube, the size of the tubes islimited to allow the heat to pass readily from the centre of the tubesto the walls where heat exchange occurs. Generally therefore the tubeshave a diameter of less than about 40 mm to ensure the required level ofheat transfer and to prevent the catalyst located towards the centre ofthe tube overheating and thermal runaway occurring. The small size ofthe tubes contributes to the high cost of construction of these reactorssince the number and hence weight of the tubes in the reactor requiredto contain a specific catalyst volume is limited.

Even at the small tube size the catalyst particles have to be relativelysmall in order to ensure reasonable mixing and heat transfer. Inaddition careful selection of conditions such as superficial velocityand gas hourly space velocity has to be made in order to maintain therequired heat transfer and manage the conversion of the reactant gasesat a reasonable overall pressure drop.

The issue of pressure drop is critical for the oxidation reaction inthat mass transfer limitation is thought to occur in catalyst pelletsleading to excess carbon dioxide production in the pores of thecatalyst. Whilst this problem can be alleviated by ensuring that theactive sites are only present near the surface of the catalyst particle,this can deleteriously affect productivity as the active sites that areavailable have to be worked hard to deliver a reasonable overallproductivity.

Various approaches have been suggested for addressing the problems ofthese reactors. One proposal is to use multizone catalyst beds. Here thefixed bed is packed with catalysts of different activities. Generallythe least active catalyst will be used at the feed point with the mostactive catalyst being located at or near the exit. Whilst this allowsfor staged heat release, packing of the catalyst into the tubes becomesa more complex, and hence more costly, procedure.

A second alternative that has been suggested is to operate with variabletemperature zones within the same catalyst bed. In this arrangement thesame catalyst bed is subjected to different temperature regimes. This isachieved by employing heat removal techniques. However, the engineeringof the reactor required to achieve this is complex and costly.

An alternative approach is to carry out the reaction in a fluid bedreactor. This does offer some advantages. In particular these reactorswill have an energy balance that provides export of high-pressure steamas a result of operating with less than half the air rate required for afixed bed system. In this arrangement, small catalyst particles aresuspended in the hydrocarbon product and are agitated by the injectionof reaction gas at the bottom of the reactor. The gas becomes highlydispersed throughout the reactor and so in theory the mass transfer areafrom gas to catalyst is very large. Additionally, as the catalystdiameter is low, the mass transfer and heat transfer resistances withinthe catalyst particle are also low. Since the catalyst surface area isrelatively large the heat transfer from catalyst particle to fluid ishigh so that the particles can be maintained at approaching fluidtemperature conditions. The high heat evolution in the reaction can bemanaged with internal or external coils in which water is vaporised.Thus in theory, carrying out the process in a bubble slurry reactoroffers various advantages.

However, problems can be encountered if the necessary particle sizedistribution in the reactor cannot be maintained while utilizing aneconomical amount of catalyst make-up.

An alternative approach has been to use a transport bed reactor. Thesereactors are also known as circulating-fluidized-bed reactors. However,these reactors still suffer from various drawbacks and disadvantages.

Thus it will be understood that whilst the various approaches tocarrying out the reactions for the production of anhydrides such asmaleic anhydride and phthalic anhydrides each offer some advantages,they also each have their own disadvantages. There is therefore still aneed to provide an improved process which addresses one or more of theproblems of prior art arrangements.

According to the present invention there is provided a process for theproduction of anhydrides by contacting a gaseous feed stream with aparticulate catalyst, said process being carried out in a tubularreactor having an inlet and an outlet, said outlet being locateddownstream of the inlet, said reactor comprising one or more tubeshaving located therein one or more carriers for said particulatecatalyst and cooling medium in contact with said at least one tube;

wherein said catalyst carrier comprises:

-   -   an annular container holding catalyst, said container having a        perforated inner wall defining a tube, a perforated outer wall,        a top surface closing the annular container and a bottom surface        closing the annular container;    -   a surface closing the bottom of said tube formed by the inner        wall of the annular container;    -   a skirt extending upwardly from the perforated outer wall of the        annular container from a position at or near the bottom surface        of said container to a position below the location of a seal;        and    -   a seal located at or near the top surface and extending from the        container by a distance which extends beyond an outer surface of        the skirt; said process comprising:    -   (a) introducing the gaseous reactants through the inlet;    -   (b) passing said reactants downwardly through said at least one        tube to the upper surface of the, or the first, catalyst carrier        where they pass into the passage defined by the inner perforated        wall of the container before passing radially through the        catalyst bed towards the perforated outer wall;    -   (c) allowing reaction to occur as the gas contacts the catalyst;    -   (d) passing unreacted reactant and product out of the container        though the perforated outer wall and then upwardly between the        inner surface of the skirt and the outer wall of the annular        container until they reach the seal where they are directed over        the end of the skirt and caused to flow downwardly between the        outer surface of the skirt and the inner surface of the reactor        tube where heat transfer takes place;    -   (e) repeating steps (b) to (d) at any subsequent catalyst        carrier; and    -   (f) removing product from the outlet.

The catalyst carrier is described in detail in PCT/GB2010/001931 filedon 19 Oct. 2010 which is incorporated herein by reference.

For the avoidance of doubt, any discussion of orientation, for exampleterms such as upwardly, below, lower, and the like have, for ease ofreference been discussed with regard to the orientation of the catalystcarrier as illustrated in the accompanying drawings. However, where thetubes, and hence the catalyst carrier, are used in an alternativeorientation, the terms should be construed accordingly.

The catalyst container will generally be sized such that it is of asmaller dimension than the internal dimension of the reactor tube intowhich it is placed. The seal is sized such that it interacts with theinner wall of the reactor tube when the catalyst carrier of the presentinvention is in position within the tube. The seal need not be perfectprovided that it is sufficiently effective to cause the majority of theflowing gas to pass through the carrier.

Generally, a plurality of catalyst carriers will be stacked within thereactor tube. In this arrangement, the reactants/products flowdownwardly between the outer surface of the skirt of a first carrier andthe inner surface of the reactor tube until they contact the uppersurface and seal of a second carrier and are directed downwardly intothe tube of the second carrier defined by the perforated inner wall ofits annular container. The flow path described above is then repeated.

The catalyst carrier may be formed of any suitable material. Suchmaterial will generally be selected to withstand the operatingconditions of the reactor. Generally, the catalyst carrier will befabricated from carbon steel, aluminium, stainless steel, other alloysor any material able to withstand the reaction conditions.

The wall of the annular container can be of any suitable thickness.Suitable thickness will be of the order of about 0.1 mm to about 1.0 mm,preferably of the order of about 0.3 mm to about 0.5 mm.

The size of the perforations in the inner and outer walls of the annularcontainer will be selected such as to allow uniform flow of reactant(s)and product(s) through the catalyst while maintaining the catalystwithin the container. It will therefore be understood that their sizewill depend on the size of the catalyst particles being used. In analternative arrangement the perforations may be sized such that they arelarger but have a filter mesh covering the perforations to ensurecatalyst is maintained within the annular container. This enables largerperforations to be used which will facilitate the free movement ofreactants without a significant loss of pressure.

It will be understood that the perforations may be of any suitableconfiguration. Indeed where a wall is described as perforated all thatis required is that there is means to allow the reactants and productsto pass through the walls. These may be small apertures of anyconfiguration, they may be slots, they may be formed by a wire screen orby any other means of creating a porous or permeable surface.

Although the top surface closing the annular container will generally belocated at the upper edge of the, or each, wall of the annularcontainer, it may be desirable to locate the top surface below the upperedge such that a portion of the upper edge of the outer wall forms alip. Similarly, the bottom surface may be located at the lower edge ofthe, or each, wall of the annular container or may be desirable tolocate the bottom surface such that it is above the bottom edge of thewall of the annular container such that the wall forms a lip.

The bottom surface of the annulus and the surface closing the bottom ofthe tube may be formed as a single unit or they may be separate piecesconnected together. The two surfaces may be coplanar but in a preferredarrangement, they are in different planes. In one arrangement, thesurface closing the bottom of the tube is in a lower plane than thebottom surface of the annular container. This serves to assist in thelocation of one carrier on to a carrier arranged below it when aplurality of containers are to be used. It will be understood that in analternative arrangement, the surface closing the bottom of the tube maybe in a higher plane that the bottom surface of the annular container.

Whilst the bottom surface will generally be solid, it may include one ormore drain holes. Where one or more drain holes are present, they may becovered by a filter mesh. Similarly a drain hole, optionally coveredwith a filter mesh may be present in the surface closing the bottom ofthe tube. Where the carrier is to be used in a non-vertical orientation,the drain hole, where present will be located in an alternative positioni.e. one that is the lowest point in the carrier when in use.

One or more spacer means may extend downwardly from the bottom surfaceof the annular container. The, or each, spacer means may be formed asseparate components or they may be formed by depressions in the bottomsurface. Where these spacer means are present they assist in providing aclear path for the reactants and products flowing between the bottomsurface of the first carrier and the top surface of a second lowercarrier in use. The spacer may be of the order of about 4 mm to about 15mm or about 6 mm deep. Alternatively, or additionally, spacer means maybe present on the top surface.

The top surface closing the annular container may include on its uppersurface means to locate the container against a catalyst carrier stackedabove the container in use. The means to locate the container may be ofany suitable arrangement. In one arrangement it comprises an upstandingcollar having apertures or spaces therein to allow for the ingress ofreactants.

The upwardly extending skirt may be smooth or it may be shaped. Anysuitable shape may be used. Suitable shapes include pleats,corrugations, and the like. The pleats, corrugations and the like willgenerally be arranged longitudinally along the length of the carrier.The shaping of the upstanding skirt increases the surface area of theskirt and assists with the insertion of the catalyst carrier into thereaction tube since it will allow any surface roughness on the innersurface of the reactor tube or differences in tolerances in tubes to beaccommodated.

Where the upwardly extending skirt is shaped, it will generally beflattened to a smooth configuration towards the point at which it isconnected to the annular container to allow a gas seal to be formed withthe annular container. The upstanding skirt will generally be connectedto the outer wall of the annular container at or near the base thereof.Where the skirt is connected at a point above the bottom of the wall,the wall will be free of perforations in the area below the point ofconnection. The upstanding skirt may be flexible.

Generally, the upstanding skirt will stop at about 0.5 cm to about 1.5cm, preferably about 1 cm, short of the top surface of the annularcontainer.

Without wishing to be bound by any theory, it is believed that theupstanding skirt serves to gather the reactants/products from theperforated outer wall of the annular container and direct them via theshapes towards the top of the catalyst carrier collecting morereactants/products exiting from the outer wall of the annular containeras they move upwardly. As described above, reactants/products are thendirected down between the tube wall and the outside of the upstandingskirt. By this method the heat transfer is enhanced down the wholelength of the carrier but as the heat exchange is separated from thecatalyst, hotter or colder as appropriate heat exchange fluid can beused without quenching the reaction at the tube wall and at the sametime ensuring that the temperature of the catalyst towards the centre ofthe carrier is appropriately maintained.

The seal may be formed in any suitable manner. However, it willgenerally be sufficiently compressible to accommodate the smallestdiameter of the reactor tube. The seal will generally be a flexible,sliding seal. In one arrangement, an O-ring may be used. A compressiblesplit ring or a ring having a high coefficient of expansion could beused. The seal may be formed of any suitable material provided that itcan withstand the reaction conditions. In one arrangement, it may be adeformable flange extending from the carrier. The flange may be sized tobe larger than the internal diameter of the tube such that as thecontainer is inserted into the tube it is deformed to fit inside andinteract with the tube.

In the present invention, the annular space between the outer surface ofthe catalyst container and the inner surface of the tube wall is small,generally of the order of from about 3 mm to about 10 mm. This narrowgap allows a heat transfer coefficient to be achieved such that anacceptable temperature difference of the order of about 10° to about 40°C. between the cooled exit gas and the coolant to be achieved.

The size of the annulus between the skirt and the catalyst wall and theskirt and the tube wall will generally be selected to accommodate thegas flow rate required while maintaining high heat transfer and lowpressure drop. Thus the process of the present invention mayadditionally include the step of selecting the appropriate size of theannulus to meet these criteria.

The process of the present invention enables relatively large reactortubes to be used. In particular, tubes having diameters in the region offrom about 75 mm to about 130 mm or even about 150 mm can be usedcompared to diameters of less than about 40 mm used in conventionalsystems. The larger diameter tubes will allow an increase in capacity ofabout 25,000 tonnes to about 250,000 tonnes a year for the same lengthreactor tube.

As discussed above the highly exothermic nature of the reactions inwhich anhydrides are formed is a major factor the design of a reactor inwhich the reaction can be carried out. The use of the catalyst carrierin the process of the present invention, allows tubes comprising aplurality of catalyst carriers to become, in effect, a plurality ofadiabatic reactors with inter-cooling.

Any suitable catalyst may be used in the process of the presentinvention. Powdered, foamed, structured, or other suitable forms may beused.

One benefit of the process of the present invention is that the carrierallows for the deployment of small diameter catalysts to be used such asthose having diameters of from about 100 μm to about 1 mm. Since theseare used in a fixed bed, the mass transfer resistances can be greatlyreduced over prior art arrangements. This will lead to improvedselectivity to the required products.

Further, as these small catalysts have a high surface area and arelocated in the direct flow of the reacting gas, they are maintained at atemperature which is very similar to that of the flowing gas. This willreduce the tendency to by-product formation.

In one alternative arrangement, a monolith catalyst may be used. In thisarrangement, the structure of the catalyst container may be modified.Full details of a catalyst container suitable for use with a monolithcatalyst is described in GB patent application no 1105691.8 filed 4 Apr.2011 the contents of which are incorporated herein by reference.

Thus according to a second aspect of the present invention there isprovided a process for the production of anhydrides by contacting agaseous stream with a monolith catalyst, said process being carried outin a tubular reactor having an inlet and an outlet, said outlet beinglocated downstream of the inlet, said reactor comprising one or moretubes having located therein one or more carriers for said monolithcatalyst and cooling medium in contact with said tubes;

wherein said catalyst carrier comprises:

-   -   a container holding a monolith catalyst, said container having a        bottom surface closing the container and a skirt extending        upwardly from the bottom surface of said container to a position        below the location of a seal and spaced therefrom, said skirt        being positioned such that there a space between an outer        surface of the monolith catalyst and the skirt; and    -   a seal located at or near a top surface of the monolith catalyst        and extending from the monolith catalyst by a distance which        extends beyond an outer surface of the skirt; said process        comprising:    -   (a) introducing the gaseous reactants through the inlet;    -   (b) passing said reactants downwardly through said at least one        tube to the upper surface of the, or the first, monolith        catalyst where they pass through the monolith catalyst;    -   (c) allowing reaction to occur as the gas contacts the catalyst;    -   (d) passing unreacted reactant and product out of the catalyst        and then upwardly between the inner surface of the skirt and the        outer surface of the monolith catalyst until they reach the seal        where they are directed over the end of the skirt and caused to        flow downwardly between the outer surface of the skirt and the        inner surface of the reactor tube where heat transfer takes        place;    -   (e) repeating steps (b) to (d) at any subsequent catalyst        carrier; and    -   (f) removing product from the outlet.

In one arrangement, the monolith catalyst is a solid, in that there issubstantially no space within the body of the monolith that is notoccupied by catalyst. When the monolith is in use in a vertical reactorwith downflow, the reactant(s) flow downwardly through the reactor tube,the reactant(s) first contacts the upper face of the monolith catalystand flows therethrough in a direction parallel to the axis of thecylinder. The seal of the container prevents the reactant(s) fromflowing around the monolith and assists the direction of the reactantsinto the catalyst. Reaction will then occur within the monolithcatalyst. The product will then also flow down through the monolith in adirection parallel to the axis of the cylinder.

Once the reactant(s) and product reach the bottom surface of thecatalyst carrier they are directed towards the skirt of the carrier. Tofacilitate this flow, feet may be provided within the carrier on theupper face of the bottom surface such that, in use, the catalystmonolith is supported on the feet and there is a gap between the bottomof the catalyst monolith and the bottom surface of the catalyst carrier.The upwardly extending skirt then directs the reactant(s) and productupwardly between the inner surface of the skirt and the outer surface ofthe monolith catalyst until they reach the underside of the seal. Theyare then directed, by the underside of the seal, over the end of theskirt and they then flow downwardly between the outer surface of theskirt and the inner surface of the reactor tube where heat transfertakes place.

In one alternative arrangement, the monolith catalyst has a channelextending longitudinally therethrough. Generally the channel will belocated on the central axis of the monolith catalyst. Thus where thereactor tube is of circular cross-section, the monolith catalyst of thisarrangement will be of annular cross-section. In this arrangement, inuse, in a vertical reactor with downflow, reactant(s) flow downwardlythrough the reactor tube and thus first contacts the upper surface ofthe monolith catalyst. The seal blocks the passage of the reactant(s)around the side of the catalyst. Since the path of flow of reactant(s)is impeded by the catalyst, it will generally take the easier path andenter the channel in the monolith. The reactant(s) then enters theannular monolith catalyst and passes radially through the catalysttowards the outer surface of the catalyst monolith. During the passagethrough the catalyst monolith reaction occurs. Unreacted reactant andproduct then flow out of the monolith catalyst though the outer surfacethereof. The upwardly extending skirt then directs reactant and productupwardly between the inner surface of the skirt and the outer wall ofthe monolith catalyst until they reach the seal. They are then directed,by the underside of the seal, over the end of the skirt and flowdownwardly between the outer surface of the skirt and the inner surfaceof the reactor tube where heat transfer takes place.

In the arrangement in which the monolith catalyst includes the channel,the catalyst carrier may include a top surface which will extend overthe monolith catalyst but leave the channel uncovered. This top surfaceserves to ensure that the reactant(s) do not enter the catalyst monolithfrom the top but are directed into the channel for radial flow.

The discussion of the specific features of the catalyst carrier above inrelation to the first embodiment applies equally in connection to thecatalyst carrier for a monolith catalyst of the second embodimentinsofar as the relevant features are present.

Whichever type of carrier is used, in one arrangement more than 40carriers, preferably more than 41 carriers are located within a singletube. More preferably, from about 70 to about 200 carriers may be used.This will enable a reasonable temperature rise of the order of fromabout 10° C. to about 20° C. to be maintained over each stage.

The radial flow through the, or each, catalyst carrier within the tubemeans that the gas flow path length is also very low when compared withprior art arrangements. Total catalyst depths of the order of about 2metres may be achieved within a tube of up to 20 metres of length atcatalyst hourly space velocities of about 2400. The low flow path meansthat the overall pressure drop achieved is an order of magnitude lowerthan that which would be experienced with the same catalyst in an axialtube not using the process of the present invention.

One advantage of being able to achieve a low overall pressure drop bythe process of the present invention is that long tubes with highsuperficial gas velocities, gases containing high quantities of inertsor a gas recycle may be accommodated without the pressure drop andpotential for catalyst crushing disadvantages experienced with highflows through current fixed bed systems. This ability to accommodaterecycle will enable overall conversion at lower per pass conversions tobe achieved at high catalyst productivity and selectivity.

The catalyst may be repeatedly and reliably loaded into the carrier at amanufacturing facility. The containers may be assembled in connectedunits which will simplify the loading of the reactor and in particularwill mean that the operators do not have to come into contact with thecatalyst. The unloading procedure is also simplified since the carriersmay be readily discharged before being taken for reprocessing.

In one arrangement of the present invention, a plurality of reactors maybe used in parallel.

Liquid product stream separate from the stream exiting the reactor willbe recovered. In the process of the present invention, unreacted gasexiting the outlet of the, or each, reactor may be further treated toremove heat. The removed heat may be reused and/or rejected to cooling.Liquid product separated from the stream exiting the reactor will berecovered.

In one arrangement, two or more reactors may be located in series fluidcommunication with facilities located between each reactor to removeheat. The heat may be reused and/or rejected to cooling. In onearrangement, unreacted feed containing steam exiting the last stage of aseries of interconnected reactors may be recycled to any suitable pointin the process. In one arrangement it will be recycled to the inlet ofthe first reactor.

In one alternative arrangement, two or more groups of parallel reactorsmay be located in series. In this arrangement, groups of parallelreactors are in series communication with facilities located betweeneach group to remove heat. The heat may be reused or rejected tocooling. In one arrangement, liquid product may be removed between eachstage with carbon oxides containing steam being passed to a subsequentreaction stage in the series. Carbon oxides containing steam exiting thelast stage of a series of interconnected reactors may be recycled to anysuitable point in the process. In one arrangement it will be recycled tothe inlet of the first reactor.

Where the process includes a plurality of reaction stages, additionalfeed may be fed to the second and/or one or more of any subsequentstages.

The process of the present invention may be used for the production ofanhydrides. In one arrangement it relates to the production of maleicanhydride from a feed stream comprising n-butane or benzene. In analternative arrangement, it relates to production of phthalic anhydridefrom a feed stream comprising o-xylene, naphthalene or mixtures thereof.

Any suitable reaction conditions may be used. In one arrangement, theinlet temperature may be in the region of about 100° C. which will givean exit temperature of about 420° C. The reaction pressure may be fromabout 0.7 bara to about 3.5 bara.

The present invention will now be described, by way of example, byreference to the accompanying drawings in which:

FIG. 1 is a schematic representation of the overall process for theproduction of anhydrides;

FIG. 2 is a perspective view from above of the catalyst carrier of thepresent invention;

FIG. 3 is a perspective view of the catalyst carrier from below;

FIG. 4 is a partial cross section viewed from the side;

FIG. 5 is a simplified diagram of the catalyst carrier of the presentinvention;

FIG. 6 is a schematic illustration of a carrier of the present inventionfrom below when located within a tube;

FIG. 7 is a schematic cross section of three catalyst carriers locatedwithin a tube;

FIG. 8 is an enlarged cross-section of Section A of FIG. 7;

FIG. 9 is a schematic representation of an alternative embodiment of thepresent invention, illustrating the flow path;

FIG. 10 is a schematic representation of a third embodiment of thepresent invention, illustrating the flow path; and

FIG. 11 is a schematic representation of the flow path between twostacked carriers of the kind illustrated in FIG. 10.

A catalyst carrier 1 of the present invention is illustrated in FIGS. 2to 4. The carrier comprises an annular container 2 which has perforatedwalls 3, 4. The inner perforated wall 3 defines a tube 5. A top surface6 closes the annular container at the top. It is located at a pointtowards the top of the walls 3, 4 of the annular container 2 such that alip 6 is formed. A bottom surface 7 closes the bottom of the annularcontainer 2 and a surface 8 closes the bottom of tube 5. The surface 8is located in a lower plane that that of the bottom surface 7. Spacermeans in the form of a plurality of depressions 9 are located present onthe bottom surface 7 of the annular container 2. Drain holes 10, 11 arelocated on the bottom surface 7 and the surface 8.

A seal 12 extends from the upper surface 6 and an upstanding collar 13is provided coaxial with the tube 5.

A corrugated upstanding skirt 14 surrounds the container 2. Thecorrugations are flattened in the region L towards the base of thecarrier 1.

A catalyst carrier 1 of the present invention located in a reactor tube15. The flow of gas is illustrated schematically in FIG. 5 by thearrows.

When a plurality of catalyst carriers of the present invention arelocated within a reactor tube 15 they interlock as illustrated in FIGS.7 and 8. The effect on the flow path is illustrated in the enlargedsection shown in FIG. 8.

A catalyst carrier 101 of a second embodiment is illustrated in FIG. 9.A bottom surface 102 closes the bottom of the container 101. Feet 103extend upwardly from the bottom surface to support a monolith catalyst104. An upstanding skirt 105 extends from the bottom surface 102. Theskirt may be corrugated and may be flattened as in a region towards thebottom surface 103.

A seal 106 is provided to extend from the monolith catalyst 104 andinteract with the wall of the reactor tube 107. Baffles 108 extendupwardly for the seal. These serve to direct flow and to separate thecarrier from the bottom surface of a carrier located above the carrier.The flow of gas is illustrated schematically by the arrows.

An alternative embodiment of the present invention is illustrated inFIG. 10. In this arrangement the monolith catalyst 104 has alongitudinal channel 109 therethrough. In this arrangement, the feet ofthe first embodiment may be omitted. This carrier is similar inarrangement to the first embodiment. However, additionally a top surface110 is provided to cover the upper surface of the monolith catalyst. Theflow of gas in the arrangement of FIG. 10 is illustrated schematicallyby the arrows.

When a plurality of catalyst carriers of the present invention arelocated within a reactor tube 107 the effect on the flow path isillustrated in the enlarged section shown in FIG. 11.

It will be understood that whilst the catalyst carriers have beendescribed with particular reference to a use in a tube of circularcross-section the tube may be of non-circular cross-section for example,it may be a plate reactor. Where the tube is of non-circularcross-section, the carrier will be of the appropriate shape. In thisarrangement, in the embodiment described in which an annular monolith isused it will be understood that the monolith will not be a circular ringand this term should be construed accordingly.

1. A process for the production of anhydrides by contacting a gaseousfeed stream with a particulate catalyst, said process being carried outin a tubular reactor having an inlet and an outlet, said outlet beinglocated downstream of the inlet, said reactor comprising one or moretubes having located therein one or more carriers for said particulatecatalyst and cooling medium in contact with said tubes; wherein saidcatalyst carrier comprises: an annular container for holding catalyst inuse, said container having a perforated inner wall defining a tube, aperforated outer wall, a top surface closing the annular container and abottom surface closing the annular container; a surface closing thebottom of said tube formed by the inner wall of the annular container; askirt extending upwardly from the perforated outer wall of the annularcontainer from a position at or near the bottom surface of saidcontainer to a position below the location of a seal; and a seal locatedat or near the top surface and extending from the container by adistance which extends beyond an outer surface of the skirt; saidprocess comprising: (a) introducing the gaseous reactants through theinlet; (b) passing said reactants downwardly through said at least onetube to the upper surface of the, or the first catalyst carrier wherethey pass into the passage defined by the inner perforated wall of thecontainer before passing radially through the catalyst bed towards theperforated outer wall; (c) allowing reaction to occur as the gascontacts the catalyst; (d) passing unreacted reactant and product out ofthe container though the perforated outer wall and then upwardly betweenthe inner surface of the skirt and the outer wall of the annularcontainer until they reach the seal where they are directed over the endof the skirt and caused to flow downwardly between the outer surface ofthe skirt and the inner surface of the reactor tube where heat transfertakes place; (e) repeating steps (b) to (d) at any subsequent catalystcarrier; and (f) removing product from the outlet.
 2. The processaccording to claim 1 wherein the catalyst has a diameter of from about100 mm to about 1 mm.
 3. A process for the production of anhydrides bycontacting a gaseous feed stream with a monolith catalyst, said processbeing carried out in a tubular reactor having an inlet and an outlet,said outlet being located downstream of the inlet, said reactorcomprising one or more tubes having located therein one or more carriersfor said monolith catalyst and cooling medium in contact with saidtubes; wherein said catalyst carrier comprises: a container holding amonolith catalyst, said container having a bottom surface closing thecontainer and a skirt extending upwardly from the bottom surface of saidcontainer to a position below the location of a seal and spacedtherefrom, said skirt being positioned such that there is a spacebetween an outer surface of the monolith catalyst and the skirt; and aseal located at or near a top surface of the monolith catalyst andextending from the monolith catalyst by a distance which extends beyondan outer surface of the skirt; said process comprising: (a) introducingthe gaseous reactants through the inlet; (b) passing said reactantsdownwardly through said at least one tube to the upper surface of the,or the first, monolith catalyst where they pass through the monolithcatalyst; (c) allowing reaction to occur as the gas contacts thecatalyst; (d) passing unreacted reactant and product out of the catalystand then upwardly between the inner surface of the skirt and the outersurface of the monolith catalyst until they reach the seal where theyare directed over the end of the skirt and caused to flow downwardlybetween the outer surface of the skirt and the inner surface of thereactor tube where heat transfer takes place; (e) repeating steps (b) to(d) at any subsequent catalyst carrier; and (f) removing product fromthe outlet.
 4. The process according to claim 1 wherein a plurality ofcatalyst carriers are stacked within the reactor tube.
 5. The processaccording to claim 1 wherein the annular space between the outer surfaceof the catalyst container and the inner surface of the tube wall isselected to accommodate the gas flow rate required while maintaininghigh heat transfer and low pressure drop.
 6. The process according toclaim 1 wherein the annular space between the outer surface of thecatalyst container and the inner surface of the tube wall is of theorder of from about 3 mm to about 10 mm.
 7. The process according toclaim 1 wherein the one or more tubes have a diameter in the region offrom about 75 mm to about 130 mm.
 8. The process according to claim 1wherein more than 41 carriers are located within a single tube.
 9. Theprocess according to claim 1 wherein from about 70 to about 200 carriersare located within a single tube.
 10. The process according to claim 1wherein a plurality of reactors are used in parallel.
 11. The processaccording to claim 1 wherein unreacted gas exiting the outlet of theeach or each reactor is treated to remove heat.
 12. The processaccording to claim 11 wherein the removed unreacted gas is reused. 13.The process according to claim 1 wherein two or more reactors arelocated in series.
 14. The process according to claim 13 wherein thereactors located in series are in fluid communication with facilitieslocated between each reactor to remove heat.
 15. The process accordingto claim 13 wherein unreacted feed containing steam exiting the laststage of the series of interconnected reactors is recycled to anysuitable point in the process.
 16. The process according to claim 15wherein unreacted feed containing steam exiting the last stage of theseries of interconnected reactors is recycled to the first reactor. 17.The process according to claim 9 wherein groups of parallel reactors arein series communication with facilities located between each group toremove heat.
 18. The process according to claim 13 wherein the heat isreused and/or rejected to cooling.
 19. The process according to claim17, wherein liquid product is removed between each group of parallelreactors with unreacted feed containing steam being passed to asubsequent reaction group in the series.
 20. The process according toclaim 20 wherein unreacted feed containing steam exiting the last stageof a series of interconnected reactors is recycled to any suitable pointin the process.
 21. The process according to claim 20 wherein the streamis recycled to the inlet of the first reactor.
 22. The process accordingto claim 1 wherein the process is for the production of maleic anhydridefrom a feed stream comprising n-butane or benzene or is for theproduction of phthalic anhydride from a feed stream comprising o-xylene,naphthalene or mixtures thereof.
 23. The process according to claim 1wherein the inlet temperature is about 100° C.
 24. The process accordingto claim 1 wherein the reaction pressure is from about 0.7 bara to about3.5 bara.